This invention relates generally to power plants, and, more specifically, to hybrid power plants including integrated fuel cells.
In certain hybrid power generation systems, fuel cells have been integrated with conventional gas turbines for increased power generation capacity in electrical power plants. Known fuel cells, such as, for example, solid oxide fuel cells include a plurality of solid fuel cells that react a gaseous fuel, such as reformed natural gas, with air to produce electrical power and a hot gas. The gas turbine compressor supplies the air for the fuel cells, which operate at elevated pressure, and the fuel cells produce hot gas for expansion in the turbine. Fuel cell stack exhaust air is combined with fuel cell stack exhaust fuel and the resulting heat release is converted to work in the turbine portion of the plant. Thus, electrical power is produced by both the solid oxide fuel cell generator and the turbine. See, for example, U.S. Pat. No. 5,413,879. Known such systems, however, are disadvantaged in several aspects.
For example, the fuel cell stacks are required to operate within narrow temperature limits that are imposed by the physical and thermodynamic processes produced therein to generate electricity. Typically a regenerative heat exchanger is used to raise the inlet air stream of the fuel cell to an acceptable temperature. The regenerative heat exchanger introduces substantial cost and complexity to the power plant that can be prohibitive in certain applications.
In addition, once an acceptable inlet temperature for the fuel cells is achieved, maintaining a uniform fuel cell stack temperature and outlet temperature often necessitates a supply of air considerably in excess of that required to chemically generate electricity in the fuel cells. Supplying this excess air to maintain uniform temperatures in the fuel cell tends to result in large compression losses. The provision of excess air tends to reduce an inlet temperature of the turbine portion of the plant and to compromise overall thermodynamic efficiency of the system.
Still further, solid-oxide fuel cells usually do not convert all of the fuel that is fed into the inlet of the fuel cells. Composition of the outlet stream from the fuel cells primarily includes CO, CO2, H2, and H2O along with the equilibrium species. In the absence of means to burn the partly spent fuel, the heat content of these constituents is wasted, thereby reducing thermodynamic efficiency of the plant. Additionally, unburned hydrocarbons may also be undesirably emitted into the atmosphere when fuel for the fuel cells in not completely converted.
It would be desirable to provide a lower cost plant with reduced emissions and increased thermodynamic efficiency.